A wide variety of portable electrical devices are available today including, but not limited to, laptop computers, personal digital assistants, mobile phones, and cordless power tools. These portable electrical devices may utilize a rechargeable battery to facilitate their portable nature. An adapter, e.g., an ACDC or a DCDC adapter, may also be available to supply power to the portable electrical device in an adapter supply mode. The rechargeable battery may be charged in the adapter supply mode. When the rechargeable battery is being charged, a DC to DC converter may accept an input DC voltage from the adapter and provide an output DC voltage and charging current to the rechargeable battery. The DC to DC converter may have one or more switches controlled by a control signal from a charger controller. The charger controller may receive various input signals representative of various power conditions and may provide an output control signal in response thereto to control the DC to DC converter.
The battery charging process may begin with a constant current charge period where the charging current to the battery is constant as the voltage of the battery rises. When the battery voltage rises to an upper voltage threshold, the charging process may enter a constant voltage charging period. These two different periods may be controlled by two different error amplifiers of the charger controller. In addition, a third error amplifier of the charger controller may be utilized to perform automatic adapter current allocation. Automatic adapter current allocation allocates available current from the adapter between a system load of the portable electrical device and the rechargeable battery. The automatic adapter current allocation ensures that the system load takes precedence over charging current to the rechargeable battery so as the system load requirements increase the charging current provided by the DC to DC converter may decrease to allow more current from the adapter to supply the system load.
Conventional loop compensation for the constant current error amplifier, constant voltage error amplifier, and adapter allocation error amplifier may utilize a resistor and capacitor pair (Rc and Cc) for each of the three error amplifiers which are external to the charger controller. This requires three separate pins on the charger controller and three separate external compensating resistor and capacitor pairs coupled to each pin. This results in additional pins, costs, and complexities for each compensating resistor and capacitor pair. Another voltage mode charger controller has each error amplifier coupled to a common node for all the error amplifiers so only one external compensation pin and one external compensating resistor and capacitor pair is necessary. However, this voltage mode charger controller still requires one integrated circuit pin and external compensating resistor and capacitor pair.
Accordingly, there is a need for eliminating such external compensation for a charger controller with its associated costs and complexities for the additional pins on the charger controller and the external compensation components.